The separation of carbon dioxide (CO2) from natural gas or NGLs is an important step in the petrochemicals industry. Carbon dioxide contamination can lower the heating value of methane and increase transportation costs. Alternatively, carbon dioxide contamination of C2 or C3 paraffin streams can lead to the formation of impurities during hydrocarbon cracking processes. Processes which sequester CO2 product streams may also be utilized in enhanced oil recovery processes.
The use of liquid extraction to scrub acid gases such as carbon dioxide (CO2) from natural gas or NGL streams is well known and typically employs an aqueous amine solvent system. Amine scrubbers work best when the CO2 levels in a hydrocarbon inlet stream are relatively low. The amine components typically employed vary widely, but the use of alkanolamines is well established. Despite their effectiveness, amine based scrubbers can be costly and difficult to operate. For example, solvent contamination is often a problem. In addition, the use of an aqueous scrubber system may cause natural gas streams or hydrocarbons to become saturated with water. Removal of water from hydrocarbon gas streams involves further purification steps (i.e. absorption by glycol systems) and further cost is added to the overall process.
In addition to the above liquid extraction systems, solid adsorbents have been explored for adsorptive uptake of carbon dioxide (CO2), ethane (C2H6), and methane (CH4). For example, the adsorption capacities of activated carbon materials at gas pressures below 1 bar typically decrease in the order C2H6>CO2>CH4 (see FIG. 1 of the article by Laukhuf, W. L. S and Plank, C. A. in the Journal of Chemical Engineering Data, 1969, 14(1), p 48.; also see Reich, R.; Ziegler, W. T. in Industrial and Engineering Process Design and Development, 1980, 19, p 336). Activated carbon materials are thus selective towards ethane (and not carbon dioxide) and are not directly useful for reducing CO2 levels in light paraffin streams. Similarly, some molecular sieves adsorb carbon dioxide less strongly than ethane, and follow the uptake capacity sequence C2H6>CO2>CH4. This is the case for ALPO-5 (see FIG. 2 in the article by Choudhary and Mayadevi in Langmuir, 1996, 12, p 980), SAPO-5 (see FIG. 3 in the article by Choudhary and Mayadevi in Langmuir, 1996, 12, p 980), and silicalite-1 (see FIG. 1 in the article by Choudhary and Mayadevi in Zeolites 1996, 17, p 501). In contrast, templated, high silica zeolites usually follow the capacity sequence CO2>C2H6>CH4, but present very low CO2/C2H6 selectivity, as it is observed for ZSM-5 (see FIG. 4 of the article by He, Y and Seaton, N. A. in Langmuir, 2006, 22, p 1150). These adsorbents cannot be used for a pressure swing adsorption (PSA) separation of CO2 from ethane, but with their nearly linear CO2 isotherms, have good opportunities for CO2/CH4 pressure swing adsorption separations. Product recovery would suffer, however, due to the adsorbent's significant CH4 capacity.
Classical aluminosilicate materials and particularly their uses in the sequestration of CO2 from feed gases comprising hydrocarbons are well represented in both the patent and academic literature. For example, aluminosilicate zeolites such as 13X are known to selectively adsorb carbon dioxide over ethane and methane (see FIG. 2 of the article by Choudhary, V. R.; Mayadevi, S.; Singh, A. P. in the Journal of the Chemical Society, Faraday Transactions, 1995, 91(17), p 2935). Similar trends have been found for NaY, Na-mordenite and 4A zeolite materials (see Breck, D. W. in Zeolite Molecular Sieves: Structure, Chemistry and Use. Wiley-Interscience Publication, John Wiley and Sons, London, 1974). However, the shape of the CO2 isotherms for 13X zeolites can vary over a wide range and are generally too steep for normal PSA processes. These materials may be more suitable for thermal swing adsorption processes.
U.S. Pat. No. 3,751,878 describes a zeolite molecular sieve for adsorbing carbon dioxide selectively from a gaseous mixture also containing methane and hydrogen. The zeolite used was a traditional porous aluminosilicate material. Several synthetic or naturally occurring zeolites were disclosed including types A, T, X, Y, S, and Z (also see: U.S. Pat. No. 3,176,445 and the references cited therein).
EP 0173501 A2 teaches the use of a faujasite type zeolite which has been ion exchanged with alkali or alkali earth metals, to achieve separation of CO2 from non-acidic gases. Non-acidic gases include carbon monoxide, nitrogen and methane. The faujasite materials within the scope of the patent were zeolites X and Y, provided they had a silicon to aluminum atomic ratio of 1.2 to 3.
U.S. Pat. No. 4,775,396 teaches selective adsorption of CO2 from methane, hydrogen or nitrogen using pressure swing adsorption. The adsorbent used was a zeolite X or Y material which was modified by cations selected from the group consisting of zinc, rare earth metals, H+or NH4+cations. U.S. Pat. No. 5,531,808 teaches a similar separation employing zeolite X, but with a silicon to aluminum atomic ratio of not greater than 1.15. With this Si:Al ratio, the adsorption of CO2 during pressure swing processes can be carried out at above 20° C. Further modifications to zeolite X materials for use in CO2 sequestration is the subject matter of U.S. Pat. No. 6,309,445. The patent teaches the use of a type X zeolite which has a silicon to aluminum ratio of less than 1.15 and has at least 75% of its exchangeable cations as potassium ions.
The use of clinoptilolite to selectively adsorb carbon dioxide from methane and other non-polar gases is taught in U.S. Pat. No. 5,587,003. Bulk separation of CO2 from methane by pressure swing adsorption where the adsorbent is naturally occurring sodium rich clinoptilolite is the subject matter of U.S. Pat. No. 5,938,819.
The adsorption of CO2, water, oxides of nitrogen and preferably acetylene from a feed gas can be affected by a mixture of zeolite and alumina as taught in U.S. Pat. No. 5,779,767. Traditional aluminosilicate zeolites are used to prepare a “fixture” with alumina.
The separation of carbon dioxide from C1 to C6 hydrocarbons using a pressure swing or temperature swing process (from approximately −50° C. to 200° C.) is taught in U.S. Pat. No. 6,024,781. The patent uses zeolite A having exchangeable cations 40-90% Na+, 10-50% K+, and 0-10% other cations, as the adsorbent material. Removal of CO2 from acetylene is exemplified.
The zeolites discussed above are natural or synthetic aluminosilicate zeolite materials. Alternatively, Engelhard has developed a new type of zeolite material comprising a family of titanosilicate zeolite materials. Titanosilicate materials are known to be useful in the adsorptive separation of nitrogen from methane (see U.S. Pat. Nos. 6,068,682 and 5,989,316). Some titanosilicate materials, such as ETS-4 and CTS-1 have pore sizes in the range of 4 and 3 Å, respectively (see U.S. Pat. Nos. 4,938,939; 5,011,591 and 6,517,611) and are commercially available from Engelhard as Molecular Gate® materials. These Molecular Gate® materials have also found use as adsorbents in pressure swing adsorption processes which separate nitrogen and/or carbon dioxide from natural gas. For example, U.S. Pat. No. 6,610,124 teaches that a cation-exchanged ETS-4 material, preferably Ba-ETS-4 may be used in an adsorption bed to selectively remove nitrogen and carbon dioxide from the C1 and C2 components of a natural gas feed stream. The patent also teaches the use of CTS-1 to effect similar separations. Molecular Gate® materials such as ETS-4 and CTS-1 exclude carbon dioxide on the basis of size, and hence facilitate a kinetic type separation. CTS-1 titanosilicate adsorbents have even been used in combination with traditional amine solvent scrubbers to reduce the CO2 levels in natural gas (see U.S. Pat. No. 7,314,503 to Syntroleum).
In contrast, ETS-10 titanosilicate materials, also developed by Engelhard (see U.S. Pat. No. 5,011,591), have large pore diameters of about 8 Å and are not expected to act as Molecular Gate® materials. Despite this, ETS-10 materials have been used to separate ethylene from mixtures of ethylene and paraffins of the same carbon number (see for example co-pending Canadian Patent application No. 2,618,267 and Al-Baghli and Loughlin in J. Chem. Eng. Data 2006, v51, p 248). The use of ETS-10 materials in the selective uptake of carbon dioxide from light paraffins has not been explored. ETS-10 materials have better thermal stability than some of their Molecular Gate® counterparts (i.e. ETS-4), and may prove to be better suited for higher temperature applications.